PHILADELPHIA -- Hosts can do their holiday guests a big favor by serving smaller portions using smaller utensils. That the word from psychologists at the University of Pennsylvania.

The findings, to be published in the journal Psychological Science, demonstrate the power of what the researchers have termed "unit bias": the sense that a particular portion of food is appropriate.  Unit bias provides the basis for understanding why portion size influences how much food you eat. Their work on how individuals decide what goes into a single serving of food offers further warning to the weight-conscious in the coming holiday season.  

"In terms of food, unit bias applies to what people think is the appropriate amount to consume, and it shows why smaller portion sizes can be just as satisfying," said Andrew B. Geier, lead author and a graduate student in Penn Department of Psychology.  "A 12-ounce can of soda and a 24-ounce bottle are both seen as single units.  But be careful, the 24-ounce bottle, though viewed as one unit, is actually more than two and a half servings of soda."

Unit bias can be seen in all types of consumption, whether it is how much food you take or how many times you ride the roller coaster.  This bias, the researchers believe, is mostly derived from a culturally designated "proper" portion.  It may also explain why portion size causes the French to eat much less than Americans.

he French eat from smaller portion sizes, but small portion size is only a barrier if there is something keeping them from consuming more portions," Geier said.  "This is where we believe unit bias comes into play; without it, people would just eat more units."

According to Geier, people see food in natural consumption units, whether that is a single wrapped candy or a plateful of food.

Geier and Penn psychology professor Paul Rozin designed their experiments to observe how people choose to act in the presence of unlimited free food in public or private settings.  In their study, they presented unsuspecting people with M&M's candies, Tootsie Rolls and Philadelphia-style soft pretzels.  When changing the size of the portions whether by offering a whole or half of a pretzel, for example people will see the offered portion as a single unit.  In the pretzel experiment, people would take and eat an entire pretzel even though they were eating twice as much as the other people who were sufficiently satisfied with a half pretzel as a single unit.

They also observed how the means of serving the portion could influence how much food is eaten.  In the M&M experiment, the researchers offered a large mixing bowl of the candy at the front desk of the concierge of an apartment building.  Below the bowl hung a sign that read "Eat Your Fill" with "please use the spoon to serve yourself" written underneath.

If presented with a small spoon, most passersby would take a single scoop, even though the sign encouraged them to take more.  If given a much larger spoon, the subjects would still take a single scoop, even though that one scoop contained much more candy.  The subjects were inadvertently eating twice as much candy when the larger scoop happened to be in the bowl.

"It is more than just people afraid of appearing greedy.  They didn't know they were being observed," Geier said.  "We have a culturally enforced 'consumption norm,' which promotes both the tendency to complete eating a unit and the idea that a single unit is the proper amount to eat."

The researchers believe a better understanding of unit bias will aid in studying the psychology of obesity.

"When we talk about overeating and obesity, we talk calories consumed and grams of fat, but we rarely mention context and environment what people see as the acceptable amount to eat," Geier said.  "There are environmental elements that dramatically affect the choices and quantities of food we consume. This is a fundamental aspect of human food choice, which is seriously understudied considering its mammoth impact on the number of calories we consume every day. "